Protocol 36.300 of Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN) describes detailed processes of handover. Since radio resources are precious, no matter handover based on X2 or handover based on S1, uplink back propagation is required in order to reduce retransmission of air interface data. The processes related to handover comprise the following steps.
After a source evolved universal mobile telecommunications system (UMTS) terrestrial radio access network (E-UTRAN) Node B (eNB) sends a handover command to a user equipment (UE), a control plane of the source eNB initiates a re-establishment command to a user plane; and the user plane re-establishes the radio link control (RLC) and the packet data convergence protocol (PDCP) instances after receiving the message. During the re-establishment process, the RLC forms data in a buffer into PDCP protocol data unit(s) (PDU) and then delivers all of them to a buffer of the PDCP, and directly discards datagram fragments if the fragments cannot be formed into PDU(s). The PDCP module processes PDUs with consecutive sequence numbers (SN) in the buffer as service data units (SDU) and then directly sends the same to a serving gateway (S-GW) of a core network via an S1 tunnel of a GTPU (general packet radio service (GPRS) tunnel protocol user plane); and the PDCP module processes PDUs with inconsecutive SNs (from a PDU with a first inconsecutive SN to the last PDU in the buffer) in the buffer as SDUs and then sends the same to a target eNB via an X2 uplink back propagation tunnel of the GTPU. After receiving the uplink back propagation datagram, a GTPU at the target eNB directly delivers it to the PDCP; and then the PDCP module stores the data in a reordering buffer. The source eNB sends a sequence number status transmission message to the target eNB, wherein the message carries loss information of the datagrams received by the source PDCP; the target eNB constructs a status report after receiving the message and then sends the status report to the UE to notify the UE which datagrams have to be retransmitted due to loss; and the PDCP module of the target eNB receives the retransmitted datagrams from the UE, then reorders the retransmitted datagrams and the datagrams in the reordering buffer, and finally sends the consecutive datagrams to the S-GW. Introduction of processes of uplink back propagation reduces repeated delivery of air interface data and saves air interface resources.
The protocol describes the processes of uplink back propagation, however, it has not specified an end time for the uplink back propagation. In the conventional art, an uplink back propagation tunnel of a source base station will be released after a resource release command from a target base station is received; and the target base station will start a timer after receiving the first uplink back propagation datagram, after the timer is timed out, it will be deemed that the transmission of uplink back propagation data ends and the uplink back propagation tunnel will be released.
The conventional art uses a most popular timer strategy, which is an empirical method. Since the timer cannot be adjusted freely during usage, it is unable to be adapted to changes of network transmission conditions and situations. If the uplink back propagation tunnel is released too early, the uplink back propagation data not transmitted completely may be lost during the transmission; whereas, if the uplink back propagation tunnel of the GTPU of the source eNB is still not released after the handover, waste of resources will be caused. Therefore, it is very important to accurately judge the end time of the uplink back propagation.